The present invention relates to a fuel metering valve forming part of an internal combustion engine supply device.
Devices of the aforementioned type are designed to supply a given quantity of fuel which is mixed with air inside the induction manifold on an internal combustion engine, and substantially comprise means for producing the said mixture and which, via appropriate ducts, are supplied with air, and, via the aforementioned metering valve, are supplied with given quantities of fuel. The said valve substantially comprises an encasement defining a circuit for the fuel fed to the valve by a supply pump, and a plug sliding axially inside the encasement between a first position, wherein it closes the said circuit, and a second position, wherein the said circuit is open. The plug is activated by an electromagnet inside the encasement and is integral with an anchor attracted by the electromagnet core.
The said encasement comprises a cap defining the front part and secured to the remaining part of the encasement itself. The said fuel circuit is formed inside the cap and terminates inside a fuel injection outlet and an axial seat guiding the plug. The said plug presents at least one pair of annular projections, the lateral surface of which consists of cylindrical surface portions and flat surface portions; the said cylindrical surface portions being designed to mate in sliding manner with the corresponding cylindrical surface on the axial seat inside the cap, in such a manner as to guide axial displacement of the plug in relation to the cap; and the said flat surface portions forming, together with the cylindrical surface of the said seat, fuel outlets forming part of the said fuel circuit. The said circuit also comprises a number of radial holes formed in the lateral annular wall of the cap and which, terminating inside the cylindrical surface of the said seat, thus communicate with the outlets formed between the said cylindrical surface of the said seat and the said flat surface portions of the annular projections on the plug.
Valves of the aforementioned type present a number of drawbacks.
Firstly, manufacture involves numerous delicate machining operations. The said holes formed in the lateral wall on the cap, and the axes of which are perpendicular to the cap axis, must be drilled successively, thus greatly increasing the total drilling time required. Furthermore, the fact that the said holes terminate inside the cylindrical surface of the plug sliding seat affects the continuity of the said surface, thus rendering grinding of the same extremely difficult, and preventing the surface diameter from being gauged easily during grinding.
Furthermore, machining the said flat surface portions (forming the fuel outlets) on the annular projections on the plug involves not only turning but also subsequent milling.
Nor is operation of the aforementioned valves wholly satisfactory, owing to fuel flow resistance and the formation of preferential fuel flow through the said outlets, caused by changes in the speed and direction of the fuel as it flows through the said outlets towards the injection outlet.
The poor grinding efficiency of the said cylindrical surfaces mating in sliding manner with the cylindrical surface of the plug sliding seat on the cap results in poor sealing between the plug and the injection outlet on the cap. In fact, owing to the presence of the said flat surface portions on the said annular projections, the said cylindrical surface portions on the said projections may be ground to a fairly wide tolerance, far lower than that obtainable by grinding a continuous cylindrical surface. Finally, on valves of the aforementioned type, the response time to a signal controlling opening and closing of the plug is fairly long, owing to the high degree of inertia on the plug itself.